The present invention relates to thermally enhanced adhesive pastes particularly well suited for bonding high density, microcircuit electronic components to substrates.
The attachment of high density, microcircuit components onto substrates, such as silicon dies onto ceramic sheet, has been an important aspect of the electronics industry for many years. Generally, it is known to use a die attach paste which is deposited between the die and substrate. Typically, the die attach paste includes a filler, an adhesive and a carrier. The filler is selected to impart to the finished bonding layer desired conductive, resistive or dielectric properties. The adhesive is chosen to create a strong bond between the die and substrate. The carrier maintains all the components in a fluid, uniform mixture, which allows the paste to be applied easily to the die-substrate interface. It also has suitable volatility to migrate from between the die and substrate following heat treatment of the assembly. After the paste is deposited and the die and substrate are assembled, the assembly is typically heated to fuse the adhesive and drive off the carrier. Upon cooling, the die is firmly attached to the substrate.
The power density of active components continues to rise, creating an increasing demand of higher thermally conductive adhesives to attach these components. These demands have previously been met by technologies described in the prior art, including U.S. Pat. Nos. 6,111,005 and 6,140,402. These patents describe a technology involving the use of powdered organic polymer resins, suspended in a non-solvent along with highly thermally conductive filler. The type of powdered resin was varied depending on the application. For large area component attachments where the Coefficient of Thermal Expansion (CTE) mismatch to the substrate was also large, low modulus thermoplastic polymers were incorporated to handle the shear stress generated at the bondline of the adhesive. For smaller area components where the expansion mismatch to the substrate was lower, thermoset or combinations of thermoplastic and thermoset polymer powders were employed in the adhesive composition with the filler. The use of the higher modulus polymers also increased the thermal conductivity.
U.S. Pat. No. 6,265,471 describes an even higher thermal conductivity technology where the highly conductive filler is suspended in a liquid epoxy resin which is dissolved in a fugitive solvent. This technology increased the thermal conductivity over the prior technology described in U.S. Pat. Nos. 6,111,005 and 6,140,402. Unfortunately, the elastic modulus of the thermosetting liquid resin system was relatively high when cured or cross-linked. Consequently, the application of this technology was limited to small area component attach and or substrates that were closely matched in CTE to the component, usually a semiconductor die. The prior art described in the technologies described above shows a linear relationship between the modulus and the thermal conductivity of the adhesive. Low modulus adhesives, described in U.S. Pat. Nos. 6,111,005 and 6,140,402, were lower in thermal conductivity, whereas the higher modulus adhesives described in U.S. Pat. No. 6,265,471 were higher in thermal conductivity. As higher function semiconductor devices grew in size and power, the need also grew for an adhesive with both high thermal conductivity and low modulus. Such adhesives were needed to absorb the bondline shear stresses caused by the thermal expansion mismatch between the die and the high expansion, high thermally conductive substrates. One large application in the marketplace is the attachment of large area, flip chip microprocessor devices to a high expansion, high thermally conductive heat spreader. Both high conductivity and low modulus properties are needed for this application. Heretofore, the adhesives described in U.S. Pat. Nos. 6,111,005, 6,140,402 and 6,265,471 were used in these applications. However, the microprocessor devices increased in power density and thus the demand increased for adhesives having even better thermal properties with low elastic modulus.